The present invention relates to electrophotographic devices, and in particular, to systems and methods for setting laser diode bias current,
In electrophotography, an imaging system forms a latent image by exposing select portions of an electrostatically charged photoconductive surface to laser light. For example, the imaging system may sweep a laser beam across the photoconductive surface in a scan direction as the photoconductive surface advances in a process direction that is substantially orthogonal to the scan direction. For each sweep of the beam, a corresponding laser is modulated to write a plurality of print elements (Pels). The density of the electrostatic charge on the photoconductive surface is altered in areas exposed to the laser beam relative to those areas unexposed to the laser beam, thus forcing a latent image on the photoconductive surface in a manner that corresponds with associated image data. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam.
The toner pattern is subsequently transferred from the photoconductive surface to the surface of a print substrate, such as paper, which has been given an electrostatic charge opposite that of the toner. A fuser assembly then applies heat and pressure to the toned substrate before the substrate is discharged from the apparatus. The applied heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the applied pressure promotes settling of the toner constituents in these voids. The toner solidifies as it cools adhering the image to the substrate.
Electrophotographic imaging systems typically utilize a semiconductor laser diode as the source of the laser beam. Semiconductor laser diodes operate with a characteristic relationship between a forward current supplied through the diode and light output from the diode. In particular, the laser diode exhibits a low current region of operation where the laser diode behaves as a light emitting diode (LED), producing primarily spontaneous emission over a relatively broad wavelength band. However, as the forward current is increased, a threshold current level is reached where stimulated emission in the device begins to dominate and the laser diode starts to behave as a laser, producing narrow-band laser light. The threshold current level for a laser diode can vary significantly from device to device, even among similar devices. Moreover, the threshold current level can vary over the normal range of operating conditions for a given device.
Imaging system electronics provided to control the laser diode typically deliver a fixed bias current level to idle the laser diode below the threshold current levels and a second switched current that is selectively applied to the laser diode to modulate the laser beam on and off. However, in many applications for which off-state power is to be minimized, the fixed bias current level must be sufficiently low that it is guaranteed not to rise above the laser diode threshold under all operating conditions and for all laser diodes that are to be utilized in a corresponding application. Because typical laser diode designs exhibit a wide range of threshold current across production units, and since the threshold current level changes as a function of laser diode temperature and age, fixed bias current sources are typically set to a current level that is generally far below the threshold current level for most laser diodes operating under nominal conditions. Thus, the fixed bias current source may be a limiting factor to the speed and flexibility of a corresponding imaging system.